Overmoulding is a technique whereby a layer of material is provided over the surface of an article by placing the already formed article in a mould and injecting the material into the mould so that it forms a layer over at least part of the surface of the article. In some instances for reasons of economy and subsequent performance of the overmoulded article it is important to be able to control the location and thickness of the overmoulded layer. Furthermore, for economy and simplicity of operation it can be desirable that the material be injected into the mould through a single injection point.
The ease with which these techniques may be performed to some extent depends upon the rheology of the material that is injected, under the conditions that it is injected and the conditions prevailing in the mould. One instance in which overmoulding is used is in the provision of a layer of foamable material on the surface of a carrier. This technique is used in the production of acoustic baffles or structural reinforcement used in the manufacture of automobiles. These overmoulded articles are mounted within the frame of the automobile which is subjected to an anticorrosion treatment such as the e-coat process and then heated in an oven to dry and bake the anticorrosion coating. The foamable layer that has been provided by overmoulding is such that it will foam at the temperatures to which it is exposed in the oven.
Accordingly it is necessary that the foamable material will flow under the injection conditions and under the conditions prevailing in the mould to produce a layer of required thickness over the desired regions of the article without foaming. Furthermore it is desirable that this can be accomplished whilst the foamable material can be injected into the mould through a single injection point. This has proved extremely difficult particularly as the size of the mouldings increases. It is also a problem when overmoulding with the structural foams used to produce structural reinforcement in automobiles such as epoxy resin and polyurethane based foams which tend to have a relatively high viscosity at the conditions under which they are injection moulded.
A further difficulty that arises with mouldings of the type previously discussed is that the article which is overmoulded can itself be formed by injection moulding and where there is a need for economic reasons to provide maximum strength with minimum use of materials such as with articles are to be used to provide structural reinforcement. Accordingly the carriers tend to be designed in lattice or honeycombed or ribbed structures to provide optimum reinforcement; the ultimate structure depending upon the position within the vehicle that the article is to be located and the nature of the reinforcement to be provided. Similarly it may be required to locate the overmoulded material on only a portion of the surface of the carrier.
The lattice, honeycomb or ribbed structure that may be required in the carrier can result in the presence of local, relatively small, interconnecting locations in the carrier. If the carrier itself is formed by injection moulding it can be difficult to control the flow of the material used to make the carrier, which can result in undesirable thick spots in the carrier at these interconnecting locations. These can be wasteful in material and can also impair the flow of material that is overmoulded on the carrier.
The present invention is therefore designed to overcome one or more of the aforementioned problems and/or overcome other problems, which will become evident to the skilled artisan from the following description.